1. Field of Invention
The present invention relates to a location scale comprising bars with a first edge and a second edge, either the first or the second edges being located at regular intervals. The concepts first edge and second edge refer to edges located closer to and further from the beginning of the scale, respectively. The reading direction of the scale, which can naturally vary, has no effect on the use of these concepts.
The invention also relates to an optical reading sensor for reading such a location scale, comprising several light receivers with different detection areas.
2. Description of the Prior Art
Location scales that are used for tracking and localization of the mutual position of two targets capable of moving with respect to each other consist conventionally of scale bars that are of uniform width and located at regular distances. Such a scale is called an incremental scale. It is read by counting the number of bars starting either from the beginning of the scale or from a separate starting signal located on the scale. The problem that arises with a scale of this kind is that every time the incremental data is lost for some reason, the device must first be driven to the starting signal on the scale before the location data can be restored. In order to avoid this problem, it is known to dispose another scale beside an incremental scale, said other scale comprising absolute codes at regular intervals for indicating the location of the code. Consequently, it is not necessary to return to the beginning of the scale to restore location data when it is lost but only to the nearest absolute code. Such scales are also known wherein the incremental code and absolute codes are mixed with each other in such a manner that a starting signal for an absolute code occurs first on the scale and thereafter the actual absolute code at regular intervals. Such a scale is disclosed, for example, in German Offenlegungsschrift No. 4,818,044. In its use, this code does not differ from the code wherein the incremental scale and the absolute codes are located on different scales in the sense that for restoring the location data, it is always necessary to return to the nearest absolute code.